High conductance cold trap for vacuum systems



Jan. 10, 1967 M. HABLANIAN 3,296,810

HIGH CONDUGTANCE COLD TRAP FOR VACUUM SYSTEMS Filed Aug. 24, 1964 United States Patent 3,296,810 HIGH CONDUCTANCE COLD TRAP FOR VACUUM SYSTEMS Marshed I-Iahlanian, Weilesley, Mass, assignor to National Research Corporation, Cambridge, Mass, a

corporation of Massachusetts Filed Aug. 24, 1964, Ser. No. 391,577 3 Claims. (Cl. 62-555} This invention relates to cryogenic baflles for gases and vapors and particularly to cryogenically cooled traps for condensing pump fluid vapors at the inlet of a large oil diffusion pump or the like.

Such bafiles are a conventional part of large vacuum systems and have been made in a variety of forms to meet the twin objectives of maximum conductance and optical tightness with respect to the pump vapors. These objectives are inconsistent and adequate removal of pump vapors is achieved at the cost of a pumping speed for the pump and baflle which is 30-40 percent of the unbaflled pump speed.

Several baffle designs are shown in my paper published in the Transactions of the Ninth Vacuum Symposium- 1962 (MacMillan Co., New York, 1963, Bancroft, ed.) at page 384 and by Pirani and Yarwood in their book, Principles of Vacuum Engineering (Reinhold, New York, 1961), pp. 53-60. See also US. Patent 3,081,068, to Milleron.

I have discovered that it is possible to achieve higher conductance in optically tight bafiles which are practical to manufacture. I have made such baffles and utilized them with a 35-inch diffusion pump with the results of effective reduction of backstreaming and retention of 50 percent of unbaflled pumping speed.

It is therefore the object of the invention to provide an improved baflie or cold trap for large vacuum systems.

Other objects and advantages of my invention will in part be obvious and will in part be set forth hereinafter.

In general, the objects of the invention are achieved by a design which incorporates a refrigerated annular shroud and additional refrigerated trapping surfaces located along what may be thought of as the diagonals of the shroud. The additional trapping surfaces consist of a central obstacle and annular semi-chevrons opening towards the central obstacle.

The invention can best be described with reference to the specific embodiment shown in the drawing, which is a schematic view of a baffle, according to the invention, mounted at the inlet of an oil diffusion pump.

The pump 10 is a 35-inch pump having a measured unbaffied pumping speed of about 49,000 liters per second. The pump is of conventional design and is commercially marketed by NRC Equipment Corporation, Newton, Massachusetts. Only the upper portion of the pump is shown in the drawing. The top jet stage 12 includes a cold cap which initially reduces backstreaming to values on the order of 10 to 5 l0- mg./cm. min. (see Power Patent 2,919,061). Backstreaming rates of this order of magnitude involve straight line motion of the backstrcaming and back migrating oil molecules with a low number of collisions.

The pump is connected to a vacuum chamber 14 which may be a large space simulation chamber or metal pro duction furnace or the like, via the battle 16 which has the form of a cylindrical tube with an inlet port 18 and an exit port 20. The exit port is disposed transversely to the tubular axis of the bafile and corresponds -to the inlet opening of pump 10. The outer wall of the baffie is cooled by circulating water in the coils 22.

The principal elements inside the shroud are a central circular disc 24, a circular shroud 26 and circular semi- ICC chevrons, 28, 3t), 32, 34 and 36. These elements are all cooled to liquid nitrogen temperature. The cooling is accomplished via coils 38, a level control tube 40 and conduction ribs 42. The ribs provide the sole support of the disc and semi-chevrons from the shroud.

The number and size of semi-chevrons is variable, subject to the following general rules: (1) they form an angle of between 50 and 20 degrees with respect to the tubular axis, 45 to 30 degrees being preferred; (2) their positioning along the diagonals leaves an annular space of triangular cross-section around the central obstacle, the triangle being formed by the locus of outer ends of the semi-chevrons and preferably substantially equilateral; (3) the semi-chevrons on the exit side of the obstacle are insufficient to afford optical tightness with respect to the exit. The central obstacle 24 has a lesser axially projected area than the exit port 20, and preferably is just sufficient to shadow the cold cap of the diffusion pump. An alternative form of the central obstacle is an oblate liquid nitrogen reservoir or a series of V central chevrons.

Shadows cast by the battle are defined by the frusto conical regions between lines AG and AG' and between lines CH and CH, the straight cylindrical regions be- 1 tween lines DG and DG and between lines FH and FH' and the intersecting cones CIC and B1B. The basic design principle is that the gaps between disc 24 and semi-chevron 34 and between the semi-chevrons are compensated by the lower ends of the chevrons 28, 30 and 32. It was observed that the principal source of backstreaming vapors is from the region of the pump cold cap. The ends of the chevrons 28, 30 and 32 are used in conjunction with the ends of chevrons 34 and 36 to minimize redundancy of trapping surfaces in view of this observation. For instance, the outer end of chevron 30 is defined by the line BH drawn from the cold cap lip through the inner end of semi-chervon 36. The inner end of semi-chevron 30 lies along the cone CIC which has the top center of the cold cap as its apex and passes through the inner end of shroud of semi-chevron 34.

The shroud 26 is located in close relation to the housing and supported therefrom by a series of insulating spacers 43. Thermal insulation is provided between the shroud and housing to minimize liquid nitrogen consumption. A portion of the insulation is indicated at 44. It has the form of multiple radiation shields consisting of layers of aluminum coated Mylar arranged as shown in the patent to I-Inilicka, 3,018,016. I have discovered that the metallized Mylar does not contribute a significant gas load to the vacuum system. Thus, it was possible to obtain vacua on the order of 10 torr while cutting liquid nitrogen consumption. According to Hnilickas teaching, the metallized Mylar is utilized in the form of crinkled sheets which make random point contacts with adjacent sheets. I have discovered that this makes an excellent creep barrier which can be run for over hours without saturation.

Other creep barriers can be used in addition to, or in lieu of, the creep barrier formed by the insulation.

Example A bafile was built according to the above embodiment for use with the above pump. The inlet port of the battle had a diameter of 54 inches and the baflie had a tube length of about 60 inches. Several tests were run to determine backstrearning through bafile. In these tests aluminum foils were disposed above the bafiies. After a pumping run the foils would be weighed, washed with acetone and then weighed again. The foils were cooled with liquid nitrogen during the runs to condense any pump vapors which might reach the foil. Backstreaming rates below 10" mg./cm. min. were achieved (a reduction of three orders of magnitude). Pump speed was measured at 24,500 liters per second.

The bafile is simple to construct and easy to service by lifting the assembly of refrigerated parts out through the top.

What is claimed is:

1. A cryogenic batfie for reducing back migration of pumping vapors from a diifusion pump or the like, characterized by retention of approximately 50% of the unbaffied speed of the pump, and comprising, in combination, a vertically arranged tubular housing having an upper inlet port and having a lower exit port transverse to the tubular axis, of smaller area than the average cross-section area of the housing, a central obstacle in the tubular body arranged along the axis thereof and having a smaller axially projected area than said exit port, a tubular shroud within the housing located in closely spaced relation to the inner wall thereof and covering a major portion of the inner wall, tubular semichevrons located within the tubular shroud and coaxial with the tubular axis of the shroud and being arranged to open towards the said obstacle and co-operating with said obstacle and shroud to define an optically tight baflie with respect to the exit port, the semi-chevrons lying susbtantially along diagonals of the tubular shroud above and below the central obstacle to leave an annular space of triangular cross-section around the said central obstacle, pointing inwardly, and means for maintaining the obstacle, shroud, and semi-chevrons at temperatures below the condensation point of the pump vapors.

2. The bafiie of claim 1 wherein the said triangular space has substantially the form of an equilateral triangle.

3. The baffie of claim 1 wherein the means for maintaining the cold temperature of the baffie structure includes low vapor pressure, radiant-barrier, thermal insulation between the shroud and housing with the insulation forming a creep barrier.

References Cited by the Examiner UNITED STATES PATENTS 2,703,673 3/1955 Winkler.

2,934,257 4/1960 Power.

3,018,016 1/-962 Hnilicka 220-10 3,044,275 7/ 1962 Drewes.

3,198,209 8/1965 Bachler.

ROBERT F. BURNETT, Primary Examiner. 

1. A CRYOGENIC BAFFLE FOR REDUCING BACK MIGRATION OF PUMPING VAPORS FROM A DIFFUSION PUMP OR THE LIKE, CHARACTERIZED BY RETENTION OF APPROXIMATELY 50% OF THE UNBAFFLED SPEED OF THE PUMP, AND COMPRISING, IN COMBINATION, A VERTICALLY ARRANGED TUBULAR HOUSING HAVING AN UPPER INLET PORT AND HAVING A LOWER EXIT PORT TRANSVERSE TO THE TUBULAR AXIS, OF SMALLER AREA THAN THE AVERAGE CROSS-SECTION AREA OF THE HOUSING, A CENTRAL OBSTACLE IN THE TUBULAR BODY ARRANGED ALONG THE AXIS THEREOF AND HAVING A SMALLER AXIALLY PROJECTED AREA THAN SAID EXIT PORT, A TUBULAR SHROUD WITHIN THE HOUSING LOCATED IN CLOSELY SPACED RELATION TO THE INNER WALL THEREOF AND COVERING A MAJOR PORTION OF THE INNER WALL, TUBULAR SEMICHEVRONS LOCATED WITHIN THE TUBULAR SHROUD AND COAXIAL WITH THE TUBULAR AXIS OF THE SHROUD AND BEING ARRANGED TO OPEN TOWARDS THE SAID OBSTACLE AND CO-OPERATING WITH SAID OBSTACLE AND SHROUD TO DEFINE AN OPTICALLY TIGHT BAFFLE WITH RESPECT TO THE EXIT PORT, THE SEMI-CHEVRONS LYING SUSBTANTIALLY ALONG DIAGONALS OF THE TUBULAR SHROUD ABOVE AND BELOW THE CENTRAL OBSTACLE TO LEAVE AN ANNULAR SPACE OF TRIANGULAR CROSS-SECTION AROUND THE SAID CENTRAL OBSTACLE, POINTING INWARDLY, AND MEANS FOR MAINTAINING THE OBSTACLE, SHROUD, AND SEMI-CHEVRONS AT TEMPERATURES BELOW THE CONDENSATION POINT OF THE PUMP VAPORS. 